This three-day ATI professional development course, Advanced Satellite Communications Systems, covers all the technology of advanced satellite communications as well as the principles behind current state-of-the-art satellite communications equipment. System design approaches discussed include Big LEO, Little LEO, DTH, Direct Radio, VSAT, commercial satellite imaging, multimedia satellite services, and IP networking over satellite. There will be course exercises and practice each day

1. Slides From ATI Professional Development Short Course
Advanced Satellite Communications System
Instructor:
Dr. John Roach
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3. Outline of Topics
I. OVERVIEW OF SATELLITE COMMUNICATIONS; HISTORY
II. SATELLITE ORBITS
III. COMM SATELLITE CHARACTERISTICS; TRANSPONDERS; TRANSPONDER USAGE TYPES:
CONNECTIVITY; MULTIPLE ACCESS METHODS
IV. COMMUNICATIONS LINK ANALYSIS
DEFINITIONS OF EIRP, G/T, Eb/No, Es/No
LINK BUDGET EQUATIONS; EXAMPLE LINK BUDGET
DEFINITIONS OF NOISE TEMPERATURE, NOISE FACTOR
ATMOSPHERIC LOSSES, INCLUDING RAIN
V. COMMON MODULATION TECHNIQUES
BPSK, QPSK, OFFSET QPSK (OQPSK)
STANDARD PULSE FORMATS, FREQUENCY SPECTRA
PSK RECEIVER DESIGN TECHNIQUES; CARRIER RECOVERY; TIMING RECOVERY
VI. OVERVIEW OF ERROR HANDLING AND ERROR CODES;
STANDARD CODES; CODING PERFORMANCE AND CODING GAIN;
VII. OVERVIEW OF SCRAMBLING & ENCRYPTION TECHNIQUES;
EFFECT ON CHANNEL PERFORMANCE
Overview 1, I-2 2/11/2011

4. Outline of Topics
VIII. EARTH STATION RF EQUIPMENT
HPAs, LNAs, FREQUENCY CONVERTERS
GAIN AND PHASE DISTORTION
HPA AM/AM, AM/PM
INTERMODULATION PRODUCTS
FREQUENCY CONVERTERS; OSCILLATOR OR PHASE NOISE
COMMUNICATIONS MODELING
IX. TDMA NETWORKS; TIME SLOTS; PREAMBLE; EXAMPLE NETWORK
X. TRANSMISSION OF TCP/IP OVER SATELLITE; USE OF PEP
XI. DVB APPROACH TO SMALL APERTURE TERMINALS; DVB-S; DVB-RCS
XII. EARTH TERMINAL ANTENNAS; POINTING, TRACKING; REGULATORY
REQUIREMENTS
XIII. SPREAD SPECTRUM TECHNIQUES; DIRECT SEQUENCE; FREQUENCY HOP;
SHORT, LONG CODES; LONG CODE ACQUISITION, TRACKING
XIV. NYQUIST SIGNALING; BANDWIDTH EFFICIENT MODULATION (BEM) TYPES
XV. CONVOLUTIONAL CODING AND VITERBI DECODING
XVI. EMERGING DEVELOPMENTS AND FUTURE TRENDS
Overview 1, I-3 2/11/2011

5. ACRONYMS
ACI Adjacent Channel Interference
ACK Acknowledgement
ACS Add-Compare-Select
AES Advanced Encryption System
AFC Automatic Frequency Control
AGC Automatic Gain Control
AJ Anti-Jam
ALC Automatic Level Control
AM Amplitude Modulation
AM/AM Ratio of AM on Output to AM on Input of an RF Device
AM/PM Ratio of PM on Output to AM on Input of an RF Device
ANIK Series of Canadian Communications Satellites
ASI Adjacent Satellite Interference
ASK Amplitude Shift Keying
APK Amplitude Phase Shift Keying
ARIANE A French Heavy Lift Launch Vehicle
ARQ Automatic Repeat Request
AWGN Additive White Gaussian Noise
BB Baseband
BCH Bose Chauhuri Hocquenheim (Block Code)
BDC Block (Frequency) Downconverter
BER Bit Error Rate
BFSK Binary Frequency Shift Keying
Overview 1, I-4 2/11/2011

6. ACRONYMS
BLOS Beyond Line-of-Sight
BoD Bandwidth on Demand
BOL Beginning of Life
BPF Bandpass Filter
BPS Bits per Second
BPSK Binary Phase Shift Keying
BSC Binary Symmetric Channel
BUC Block (Frequency) Upconverter
BW Bandwidth
C Band Frequency Band from 4 GHz to 6 GHz
CBR Carrier-Bit Recovery (Intelsat TDMA Header Segment)
CCIR Comite Consultatif International des
Radiocommunications (now replaced by ITU-R)
CCITT Comite Consultatif International Telegraphique et
Telephonique (now replaced by ITU-T)
CDC Control and Delay Channel (Intelsat TDMA Header
Segment)
CDMA Code Division Multiple Access
CEPT Conference Eurpeene des Postes
CEVD Convolutionally Encoded-Viterbi Decoded
C/I Carrier to Interference Ratio
C/IM Carrier to Intermodulation Product Ratio
C/kT Carrier to Noise Density Ratio
CMA Control, Monitor, and Alarm
Overview 1, I-5 2/11/2011

7. ACRONYMS
C/N Carrier to Noise Ratio
C/No Carrier to Noise Density Ratio
CNR Carrier to Noise Ratio
CODEC Coder/Decoder
COMSAT Communication Satellite Corporation
COTM Communications-on-the-Move
CPE Customer Premises Equipment
CPFSK Continuous Phase Frequency Shift Keying
CPSK Coherent Phase Shift Keying
CSC Control and Signaling Channel
CVSD Continuously Variable Slope Delta Modulation
DA Demand Assignment
DAMA Demand Assignment Multiple Access
dB Decibel
dBi Decibel with respect to Isotropic
dBm Decibel with respect to 1 Milliwatt
DBS Direct Broadcast Satellite
dBW Decibel with respect to 1 Watt
D/C Frequency Downconverter
DEMOD Demodulator
DEMUX Demultiplexer
DE Differentially-Encoded,
DES Data Encryption Standard
DL Downlink
DM Delay Modulation
DMC Discrete Memoryless Channel
Overview 1, I-6 2/11/2011

8. ACRONYMS
DC (Frequency) Down Converter
DS Direct Sequence (CDMA spreading technique)
DPSK Differential Phase Shift Keying
DQPSK Differential Quadrature Phase Shift Keying
DSB-SC Double Sideband-Suppressed Carrier
DSCS Defense Satellite Communication System
DSI Digital Speech Interpolation (Intelsat terminology)
DVB-RCS DVB-Return Channel by Satellite
DVB-S Digital Video Broadcasting-Satellite
DVB-S2 Digital Video Broadcasting-Satellite, Generation 2
Eb/No Energy per Bit to Noise Density Ratio
ECC Error Correction Coding
EDAC Error Detection and Correction
EHF Extra High Frequency
EIRP Effective Isotropically Radiated Power
EOL End of Life
EMP Electromagnetic Pulse
ES Earth Station
ESA European Space Agency
Es/No Energy per Symbol to Noise Density Ratio
ET Earth Terminal
FCC U.S. Federal Communications Commission
FDM Frequency Division Multiplex
FDMA Frequency Division Multiple Access
Overview 1, I-7 2/11/2011

9. ACRONYMS
FEC Forward Error Correction
FET Field Effect Transistor
FFH Fast Frequency Hop
FFSK Fast Frequency Shift Keying
FH Frequency Hop
FL Forward Link (VSAT or DVB terminology)
FOM Figure of Merit
FM Frequency Modulation
FSK Frequency Shift Keying
GEO Geosynchronous Earth Orbit
GHz Gigahertz
G/T Ratio of Antenna Receive Gain to Noise Temperature
HEO Highly Elliptical Orbit
HF High Frequency (3-30 MHz)
HP Horizontal Polarization
HPA High Power Amplifier
HPF High Pass Filter
Hz Hertz
IBO Input Backoff
IC Integration Contractor
IF Intermediate Frequency
Overview 1, I-8 2/11/2011

10. ACRONYMS
IFL Interfacility Link
INMARSAT International Maritime Satellite Organization
INTELSAT International Telecommunications Satellite Organization
IOT In-Orbit Test
IP Internet Protocol
IPA Intermediate Power Amplifier
ISL Inter-Satellite Link
ITU International Telecommunications Union
K Kelvin, unit of temperature with respect to –2730C
K-Band 10-30 GHz
Ka-Band 15-30 GHz
KBPS or Kb/s Kilobits per Second
KHz Kilohertz
KPA Klystron Power Amplifier
Ku-Band 10-15 GHz
KW Kilowatts
L-band 1-2 GHz
LEO Low Earth Orbit
LHCP Left Hand Circular Polarization
LNA Low Noise Amplifier
LO Local Oscillator
LOS Line-of-Sight
LPD Low Probability of Detection
LPF Low Pass Filter
LPI Low Probability of Intercept
Overview 1, I-9 2/11/2011

11. ACRONYMS
LSB Least Significant Bit
M&C Monitor and Control
MA Multiple Access
MAP Maximum a Posteriori
ML Maximum Likelihood
MLD Maximum Likelihood Detector
MLSE Maximum Likelihood Sequence Estimator
MLSR Maximum Length Shift Register Sequence
MBPS or Mb/s Megabits per Second
MCPS or Mc/s Megachips per Second
MEO Medium Earth Orbit
MF Matched Filter
MFSK M-ary Frequency Shift Keying
MHz Megahertz
MILSTAR U.S. Military Satellite System
MODEM Modulator-Demodulator
MSB Most Significant Bit
MSK Minimum Shift Keying
MUX Multiplexer
mw Milliwatt
MW Megawatt
NAK Negative Acknowledgement
NB Narrow Band
Overview 1, I-10 2/11/2011

12. ACRONYMS
NBW Noise Bandwidth
NEO Near Earth Object
NF Noise Figure
NLOS Non-Line-of-Sight
NRZ Non Return to Zero
OBO Output Backoff
OD Orbital Debris; Orbit Determination
OQPSK Offset-QPSK
OMT Orthomode Transducer
OW Order Wire
PA Power Amplifier
PCM Pulse Code Modulation
P/D Power Divider
PEP Performance Enhancing Proxy
PG Processing Gain
PLL Phase Lock Loop
PM Phase Modulation
PN Pseudo-noise (sequence)
PR Partial Response Signaling
PRN Pseudo-random Noise
PSD Power Spectral Density
PSK Phase Shift Keying
QAM Quadrature Amplitude Modulation
QPR Quadrature Partial Response
Overview 1, I-11 2/11/2011

13. ACRONYMS
QPSK Quadrature Phase Shift Keying
RA Random Access
RF Radio Frequency
RHCP Right Hand Circular Polarization
RL Return Link (VSAT or DVB terminology)
RMS Root-Mean-Square
RS Reed-Solomon Code
RSL Received Signal Level
RSS Root Summed Square
Rx Receiver
RZ Return to Zero
S-Band 2-4 GHz
S/C Spacecraft
SC Service Channel (Intelsat TDMA Header Seqment)
SCPC Single Channel per Carrier
SFD Saturation Flux Density
SFH Slow Frequency Hop
SHF Super High Frequency
SIT Satellite Interactive Terminal
SNMP Simple Network Management Protocol
SNR Signal to Noise Ratio
SOTM SATCOM-on-the-Move
SPADE Intelsat SCPC System
SQPSK Staggered QPSK
Overview 1, I-12 2/11/2011

14. ACRONYMS
SS Spread Spectrum
SSMA Spread Spectrum Multiple Access
SSPA Solid-State Power Amplifier
SW Switch
TCP Transmission Control Protocol
TDM Time Division Multiplexing
TDMA Time Division Multiple Access
TDRS Tracking and Data Relay Satellite
TDRSS NASA Tracking and Data Relay Satellite System
TPC Turbo Product Code
TT&C Tracking, Telemetry, and Commanding
TWTA Traveling Wave Tube Amplifier
Tx Transmitter
U/C (Frequency) Upconverter
UDP User Datagram Protocol
UHF Ultrahigh Frequency
UL Uplink
USAT Ultra Small Aperture Terminal
UW Unique Word (Intelsat TDMA Header Seqment)
VA Viterbi Algorithm
VCO Voltage Controlled Oscillator
VCXO Voltage Controlled Crystal Oscillator
VP Vertical Polarization
Overview 1, I-13 2/11/2011

15. ACRONYMS
VSAT Very Small Aperture Terminal
W Watts
WB Wideband
WG Waveguide
X-Band 7-8 GHz
Overview 1, I-14 2/11/2011

16. I. Overview and History
Overview 1, I-15 2/11/2011

17. Early History of Satellite Communications
1945 - Arthur C. Clarke wrote about extraterrestrial relays
Passive Reflectors (uplink signals reflected back to earth):
1951 - Bounce off the moon
1960/64 - Bounce off US-launched 100’ &135’ diameter Echo mylar balloons
1963 - Bounce off Project Westford dipoles in orbit
Active Satellites:
1957 - Russian Sputnik 1- Launch 10/4/1957 – no mission payload
1958 - Explorer I – JPL – measured cosmic rays, etc; Launch: 1/31/1958
1958 - Project Score - US DoD– Launch: 12/18/1958; world’s first
communications satellite
- Recorded Christmas message on tape recorder; UHF
- First store-&-forward and real time communications
- Battery-powered
- 185 x 1484 km orbit; 32.3° Inclination
1960 - Courier 1B - US DoD; Launch: 10/4/1960
- First with solar cells & nickel cadmium batteries
- 1 Voice channel & TTY, 2 Watts, 1.7 - 1.9 GHz
- Orbit altitude: 938 x 1237 km orbit; 28.33° inclination
Overview 1, I-16 2/11/2011

18. Early History of Satellite Communications (cont’d)
1962 TELSTAR I (AT&T) 1968 - INTELSAT III
- First publically available
instantaneous repeating satellite 1969 - TACSAT I (DOD)
- 6/4 GHz, 3.3 Watts
- First live TV transmission across the Atlantic 1971 - INTELSAT IV
- 600 One-way voice circuits - 12 36 MHz channels;
- Demonstrated large earth terminal antennas 6 Watts/channel
1962 - Relay (NASA/RCA) 1972 - ANIK (Canada)
- Two 10 Watt transponders (4630 x S.M.; 47.5°) - First domestic satellite
1963 - Syncom II (NASA) 1974 - WESTAR (Western Union)
- First geosynchronous satellite - First U.S. domestic satellite
- Two channels, 500 kHz each
- 7.31/1.8 GHz, 2 Watts 1977 - Advanced WESTAR/TDRSS
- Inclination: 32° (II) 0.5° (III) - Commercial s/c development with
Western Union financing guaranteed
1965 - Molniya (USSR) by NASA
- Elliptical orbit
- Inclination angle: 63.4° 1993 - ACTS (NASA)
- First Ka technology satellite
1965 - Intelsat I (Early bird)
NASA then turned SATCOM technology
1966 - Intelsat II development over to industry
Overview 1, I-17 2/11/2011

19. General Satellite System Architecture
USER
SATELLITE SATELLITE
USER TERMINAL
CONTROL HUB NETWORK
TERMINAL
CENTER EARTH OPERATIONS
(TT&C) STATION CENTER (NOC)
DATA SOURCE
Handset/LAN Handset/LAN
1
Overview 1, I-18 2/11/2011

20. Satellite System Operators
• Example Satellite System Operators:
– SES
– Intelsat
– Eutelsat
– PanAmSat
– JSAT
– Telesat Canada
– Space Communications (Japan)
– Loral Space and Communications
– Many other European and Asian operators
• Each analyzes requests for service to assure legal and efficient use
• Each protects their users
– Operators cooperate to protect each others systems
– Continuously monitor and control use of their satellites
– Help investigate/characterize/geolocate interference sources
• One equipment supplier claims to be capable of locating an interference
to within 10 km from GEO
Overview 1, I-19 2/11/2011

21. Legal Authorities over Spectrum
• International Telecommunications Union (ITU):
– Controls RF frequency assignments worldwide
– Controls orbit locations (e.g., longitude for GEO) for satellites
– Also has provided many technical standards for use in
• SATCOM
• Other radio environments, e.g., microwave LOS radios
• In-Country Governmental Regulatory Body:
– Controls spectrum use within the country
– United States:
• Federal Communications Commission (FCC) manages non-government use
• National Telecommunications and Information Administration (NTIA) manages Federal
use of spectrum
– Foreign countries: Formerly, the PTT (which was a part of government) typically also
managed radio spectrum. Varies country to country
• Purpose:
– Protect customers and assures efficient use of spectrum
• Provides legal protection from interference from users on their systems or other systems
• Satellite operators, e.g., work together to assure limited inter-system interference
• Procedure:
– Users must typically coordinate with other systems and obtain license
before beginning operations – outside the US, this is often termed obtaining
“landing rights”
Overview 1, I-20 2/11/2011

22. Major US SATCOM RF Frequency Bands
RF Band Bandwidth Downlink Uplink
UHF: Military 5 & 25 KHz 243–270 MHz 292-319 MHz
C Band: (6/4 GHz) 500 MHz 3.7-4.2 GHz 5.925-6.425 GHz
X Band (8/7 GHz) 500 MHz 7.25-7.75 GHz 7.9-8.4 GHz
Ku Band: 14/11 GHz 500 MHz 10.95-12.75 GHz 13.75-14.5 GHz
Ka Band: 30/20 GHz
Commercial Ka 2.5 GHz 17.7- 20.2 GHz 27.5-30 GHz
Military Ka 1 GHz 20.2-21.2 GHz 30-31 GHz
Military EHF: (44/20) 2 GHz up/1 down 20.2-21.2 GHz 43.5-45.5 GHz
Military RF Inter-Satellite Band: 5 GHz 59-64 GHz
Note: The exact RF band range for satellite use varies beween the three ITU Regions
(US is in Region 2) & standard vs. extended band.
The above are general band designations used throughout industry and in this course.
These SATCOM band designations were adopted from radar band designations
Overview 1, I-21 2/11/2011

23. Major Advantages of Satellite Communications
• Interconnects users distributed across wide geographical areas
• Provides access for rural users with limited local terrestrial communications
• Easily supports broadcast to many terminals simultaneously
• Based on satellite antenna footprint
• Provides reasonably wide bandwidths
• User terminals can be installed very quickly
•Transportable/mobile terminals valuable for
• Disaster support & recovery
• Satellite Newsgathering (SNG trucks)
• Early Deployment of troops in foreign areas
• Transit case (TC) terminals can be checked as baggage on airplanes
Overview 1, I-22 2/11/2011

24. Current C/Ku Satellite Antenna Footprints
Support Wide Connectivity Among Users
Overview 1, I-23 2/11/2011

25. SATCOM Disadvantages and Potential Remedies
• Most communications satellites are in 23,000 miles high GEO orbits
• Relatively large link signal loss and long transmission delay
• Potential solutions:
• Use large antennas and high power amplifiers
• Use Performance Enhancing Protocols (PEPs) for TCP/IP links
• Heavy rainfall causes link fading particularly at Ku/Ka RF bands
• Potential solutions:
• Use additional link margin
• Use adaptive link data rate, or adaptive coding/modulation
• Use site diversity
• Interference can be a issue:
• Co-channel interference due to operator errors
• Other user is pointed at wrong satellite, on incorrect RF frequency
or polarization
• Adjacent satellite interference (ASI) from users with very small
antennas
• Potential solutions:
• Work with satellite operator and NOC to determine source of
interference and depend on operator to police your link per your lease
agreement
Overview 1, I-24 2/11/2011

26. Typical Transponder Services and Protection
• Example commercial satellite offerings:
– Full period, 24/7
• Monthly
• Yearly
• Multi-year – the longer the period, the lower the cost
– Scheduled & recurring e.g., at 2-3 PM EST every day
– Occasional use
• Good example is a Satellite Newsgathering (SNG) truck
• Example levels of protection available for full time service:
– Fully protected: in the event of transponder failure, protection of users by
• Assignment of other pre-emptible transponders – same satellite
• Assignment of other pre-emptible transponders – other satellites
– Non-Pre-emptible: Cannot be pre-empted in case of other transponder failures
– Pre-emptible: not protected ---- could be pre-empted in case of other
transponder failures
Overview 1, I-25 2/11/2011

27. II. Satellite Orbits
An Excellent Reference:
Roger Bate, Donald Mueller, Jerry White,
Fundamentals of Astrodynamics, Dover Publications, 1971
Overview 1, I-26 2/11/2011

28. Classes of Satellite Orbits
• Low Earth Orbit (LEO) --- defined as having altitude < 2000 km
– Circular, e.g., Iridium, Globalstar, Orbcomm; also many scientific, weather spacecraft
– For comm use, a constellation of satellites is usually required to achieve reasonable
visibility to users
– A number of standard constellations of multiple satellites have been defined to meet
certain objectives:
• Walker constellations, etc.
• Usually specified as, e.g., 7 spacecraft in each of 9 orbital planes at a specified
inclination angle, equally spaced around the equator
• Medium Earth Orbit (MEO)
– Circular, with altitudes from ~ 2,000 km out to 35786 km,
– e.g., GPS is ~ “half-synchronous” with altitude of ~ 20,200 km
– Not many communications satellites in this regime; also Van Allen belts are in MEO
• Highly Elliptical Orbit (HEO)
– Elliptical orbits, e.g., Molniya, Tundra, primarily at 63.40 inclination
– Achieves good visibility with high average elevation angles for users at high latitudes
• Geosynchronous Earth Orbit (GEO)
– Circular, with altitude such that the orbital period exactly equals one sidereal period of
the earth’s rotation
– If excellent station-keeping is maintained, this could be called a “geostationary” orbit
– By far the dominant orbit for communications satellites
Overview 1, I-27 2/11/2011

29. General Cases of Orbital Geometry
Overview 1, I-28 2/11/2011

30. Common Names for Circular Orbits
Overview 1, I-29 2/11/2011

31. Three Parameters Describe Orbit Size and Shape
1
 b 2 2
e = 1 − 2 
 
 a 
b
η
Apogee Perigee
Earth
a
• Semi-Major axis, a
• Eccentricity, e
• True Anomaly, η
Overview 1, I-30 2/11/2011

32. Three Parameters Describe Orbit Orientation
• Angle of Inclination, I
• Angle between orbital and equatorial planes
• Right ascension of ascending node, Ω
• Measured eastward in the equatorial plane from the vernal equinox
• Argument of Perigee, ω
• Measured in orbital plane in the direction of the orbital motion from ascending node
to line from earth center to perigee
Overview 1, I-31 2/11/2011

33. Communications-oriented Characteristics of
Circular Orbits
• Orbital Period
• Coverage
• Time above the Horizon
Overview 1, I-32 2/11/2011

34. Orbital Period for a Circular Orbit
• From Kepler’s Laws we know that the orbital sidereal period is a function of
satellite altitude:
T(sec) = [2π/√GMe](re + h)3/2
where re is the earth’s radius, h is the satellite altitude, and
GMe = 398,600.4418 km2/s2
• Note that: re = 6378 km (or 3444 nm or 3963 sm)
• For example, the period of a satellite whose altitude is zero (the so-called
Herget orbit -- the absolutely minimum orbital period possible) would be:
TH = 84.486 minutes
Overview 1, I-33 2/11/2011

35. Sidereal Period vs. Low Altitude Satellite
130
120
Sidereal Period (min)
110
100
90
SKYLAB
80
0 100 200 300 400 500 600 700 800 900 1000
Altitude (nmi)
Overview 1, I-34 2/11/2011

36. Sidereal Period vs. High Altitude Satellite
1600
1400
Synchronous
Orbit
1200
Sidereal Period (min)
1000
800
600
400
200
10,000 11,000 12,000 13,000 14,000 15,000 16,000 17,000 18,000 19,000 20,000
Altitude (nmi)
Overview 1, I-35 2/11/2011

37. Maximum Visibility (Zenith Pass) for a LEO
30
25
20
Time in View (min)
15
10
SKYLAB
5
0
0 100 200 300 400 500 600 700 800 900 1000
Altitude (nmi)
Overview 1, I-36 2/11/2011

38. Maximum Visibility for a MEO/GEO Satellite
35,000
30,000
20 Days
25,000
Time in View (min)
20,000
15,000 10 Days
Synchronous
Orbit
10,000
5,000
0
10,000 11,000 12,000 13,000 14,000 15,000 16,000 17,000 18,000 19,000 20,000
Altitude (nmi)
Overview 1, I-37 2/11/2011

39. Example LEO Ground Trace: ISS (278<h< 460 km)
Overview 1, I-38 2/11/2011

40. Example HEO Ground Trace: Molniya (e = 0.72)
Overview 1, I-39 2/11/2011

41. “Station Circle” Size Depends on LEO Altitude &
Minimum Allowed Terminal Elevation Angle
Overview 1, I-40 2/11/2011

42. Implications of the Station Circle Geometry for LEOs
• Only users who are both within a station circle are able to simultaneously
communicate directly via a LEO
– Very limited geographic coverage for real time communications
– Potential solutions for communicating using LEOs
• Use store-and-forward techniques
– Uplinked message is stored on board S/C and rebroadcast downlink when
intended receiver comes into view
• DoD (DARPA) had several LEO S/C in orbit at the start of First Gulf War (1991) and
experimented with this approach:
• MAC I, MAC II; MICROSAT – single plane of 6 small satellites
• Use crosslinks between LEOs to relay the messages
– Iridium took this approach
• Implement lots of ground sites for coverage
– Globalstar took this approach
• Many environmental & scientific satellites are in LEO orbit due to
their sensor requirements and must communicate in one of two main
ways:
– Store telemetry and mission data as needed
• Burst telemetry and mission data down and receive commands when their
ground station(s) come into view
– Use the Tracking & Data Relay Satellite System (TDRSS) as a GEO
relay
Overview 1, I-41 2/11/2011

43. Spacecraft Velocity Depends on Orbit Altitude
• The linear velocity of a spacecraft in circular orbit can be found from
the circumference of the orbit and the orbital period:
v = 2π(re+h)/T
= 631.35/(re+h)1/2
• So, for a 300 km orbit, v = 7.73 km/sec
• For a GEO orbit, (re+h= 42,223 km), v = 3.07 km/sec
• The velocity of a spacecraft in an elliptical orbit at perigee can be
higher than the velocity of a spacecraft in the lowest circular orbit, or
as high as ~ 10 km/sec
Overview 1, I-42 2/11/2011

44. Geosynchronous Orbits
Overview 1, I-43 2/11/2011

45. Geosynchronous Satellite Orbit Altitude
From Kepler’s Law:
T = [2π /(GMe )1/2](re + h)3/2
where re is the earth’s radius, h is the satellite altitude, and
GMe = 398,600.4418 km2/s2 (typically designated as µ)
Thus for a satellite whose orbital period is equal to 1 sidereal day
(23 hours, 59 minutes, 4 seconds, or 86,344 sec)
re + h = 42,223 km or 23,093 nm, or 26,242 sm
Thus the altitude of a Geosynchronous satellite is
h = 42,223 km – 6378 km = 35845 km
or
h = 23,093 nm - 3444 nm = 19,649 nm
or h = 26,242 sm – 3963 sm = 22,279 sm
Overview 1, I-44 2/11/2011

46. “3-Ball” GEO Constellation and its Geometric Coverage
But spacecraft antenna footprints will determine actual coverage
Overview 1, I-45 2/11/2011

47. Potential GEO Coverage Varies Slightly with
Terminal’s Elevation Angle
Overview 1, I-46 2/11/2011

48. Perturbations from the Ideal GEO Orbit
• There are three major perturbations (plus other smaller influences)
that require expenditure of ∆v for station-keeping:
• Gravitational action of the moon and the sun
• Earth’s triaxiality (non-sphericality)
• Solar radiation pressure
• The major perturbation is the precession of the orbital plane causing it to
increase over 26 years to about 150 before returning back to 00
• Inclination can increase at about 0.850 initially
• “Gravity wells” exist due to the ellipticity of the equator that would affect a
GEO E-W station-keeping that should be corrected:
• A GEO would tend to move toward one of the two stable points at:
– 75.30 E and 104.70 W (Himalayas and Rockies)
• A GEO would tend to move away from one of the two unstable points at:
– 165.30 E and 14.70 W (Marshall Islands and Portugal)
• Station-keeping fuel necessary to maintain the assigned longitude and to
minimize inclination angle of the orbital plane can add 10-40% to the dry
mass of a GEO; fuel is measured in units of change in velocity, ∆v.
• N-S station-keeping requires ∆v ~50 m/s per year; E-W up to 2 m/s per year
• Some satellites have also been launched with ion-thrusters
Overview 1, I-47 2/11/2011

49. “Figure 8” Movement of a GEO Satellite
• If a GEO’s orbital plane has an inclination angle with respect to the equatorial plane
equatorial plane ≠ 0, then its subsatellite point on the earth’s surface, traces out a
figure 8 pattern on the earth’s surface:
• The figure 8 repeats every 24 hours
• The N-S dimension of the figure 8 increases as the inclination increases
• The sun and moon would cause an uncorrected GEO to increase up to ~150
inclination
• Example Figure 8 ground trace of the nadir point:
Overview 1, I-48 2/11/2011

50. Inclined Orbit Operations
• Since station-keeping fuel is the major determinant of normal
spacecraft operational life, one approach to extending the
operations of a GEO is to:
– Reduce expenditure of station-keeping fuel by not correcting
(very often) for orbital inclination angle and allow figure 8
movement to increase
– Allow inclination angles up to 3-50 or more
– Use of the patented “Comsat Maneuver” made Inclined Orbit
Operations feasible by compensating for footprint movement
with changing motion of the spacecraft
• Major drawback is that earth station antennas (with narrow
beamwidths) would be required to track the satellite’s movement
– However the slow Figure 8 movement over a 24 hour period
can be tracked by relatively inexpensive tracking systems
Overview 1, I-49 2/11/2011

51. Orbital Debris
Overview 1, I-50 2/11/2011

52. One Representation of Only the Largest Space Objects
Overview 1, I-51 2/11/2011

53. Overview of the Orbital Debris Problem
• Since Sputnik there have been ~ 5000 space missions
• Number of debris fragments > 1 cm size estimated to be > 500,000
• Total objects now officially cataloged by the DoD is ~ 34,000, of which
~ 13,000 are still in orbit + ~ 5,000 that are being tracked but not cataloged
– The unpredicted collision of Iridium 33 and COSMOS 2251 in February
2009 resulted in the addition of > 1500 large (> 10 cm) pieces of debris
• Concentrated near 800 km but extending from 200-1700 km
• Approximately 1300 objects are satellites but only ~ 800 have fuel and can
be moved if necessary to avoid a collision
• Space Surveillance Network can track objects larger than ~ 10 cm in LEO
orbit up to ~ 2000 km altitude
– 10 cm debris at 5-7 km/sec can do terrific damage
– The next generation Space Fence is required to track up to 200,000
objects in LEO orbit vs. current Space Fence tracking of 20,000 objects
• Other sensors can do much better than 10 cm but are not available full time
– E.g., NASA Solar System Radar at Goldstone (70m) can detect
mm-size debris in LEO orbit
• At GEO, the minimum estimated size routinely tracked is ~ 70 cm
Overview 1, I-52 2/11/2011

54. Histogram of Cataloged Objects as of 5 June 2009
Overview 1, I-53 2/11/2011

55. US Government Guidelines for Disposal
• Operational lifetime limited to 25 years
• Spacecraft or upper stage must be disposed of by one of three
methods:
– LEO Orbits: Atmospheric Reentry Option
• Maneuver to orbit in which, using conservative projections for solar activity, atmospheric
drag will limit lifetime to < 25 years after completion of mission; risk of human casualty
should be
< 1 in 10,000
– “Storage” Orbit
• Between LEO and MEO: Maneuver to an orbit with perigee altitude above 2000 km and
apogee altitude below 19,700 km ( 500 km below semi-synchronous, e.g., where GPS
is)
• Between MEO and GEO: Maneuver to an orbit with perigee altitude above 20,700 km
and apogee altitude below 35,300 km (500 km above semi-synchronous and 500 km
below synchronous altitude
– GEO: See next slide
• Direct Retrieval: Unlikely with current technology
Overview 1, I-54 2/11/2011

56. Inter-agency Space Debris Coordination (IADC)
Committee Guidelines on GEO Disposal
• A GEO should retain enough fuel to be maneuvered into an orbit above the
GEO protected region fulfilling the following two conditions:
– A minimum increase in perigee altitude of
235 km + (1000 x CR x A/m), where
CR is the solar radiation pressure coefficient
A/m is aspect area to dry mass ratio (m2/kg-1)
235 km is the sum of :
200 km (upper altitude of GEO protected region) &
35 km (max. descent of re-orbited s/c due to luni-
solar & geopotential perturbations)
– An eccentricity of ≤ 0.003 (added in 2007)
• Bottom line: 300 km above nominal GEO altitude is typically used
• In addition: Operators should passivate all spacecraft stored energy
sources:
– Chemical: vent chemicals, burn excess fuels, relieve pressure vessels
– Electrical: discharge batteries
Overview 1, I-55 2/11/2011

57. To learn more please attend ATI course
Advanced Satellite Communications Systems
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